[.travis.yml] Fix Tavis CI build error
[.travis.yml] Fix Tavis CI build error

Travis CI recently updated the default distribution from Ubuntu
Precise to Ubuntu Trusty, which causes all builds except nightly
to fail.

For unknown reasons phpcs is unable to locate "PHP/CodeSniffer/
autoload.php" causing it to fail with a fatal error

The root cause of the failure is unknown. We explicitly return to
the previous build system (Ubuntu Precise) for builds to work again.

See the migration guide for reference:
https://docs.travis-ci.com/user/precise-to-trusty-migration-guide/

Notice: Ubuntu Precise is retired as of September 2017 and will be
decommissioned in the near future:
https://docs.travis-ci.com/user/reference/overview/

Riduidel/rss-bridge,kranack/rss-bridge,teromene/rss-bridge,kranack/rss-bridge,simonbru/rss-bridge,sebsauvage/rss-bridge,Riduidel/rss-bridge,kranack/rss-bridge,ORelio/rss-bridge,Frenzie/rss-bridge,simonbru/rss-bridge,RSS-Bridge/rss-bridge,kranack/rss-bridge,teromene/rss-bridge,ORelio/rss-bridge,GregThib/rss-bridge,Riduidel/rss-bridge,ORelio/rss-bridge,RSS-Bridge/rss-bridge,RSS-Bridge/rss-bridge,Frenzie/rss-bridge